Directional couplers are among very important components for RF/microwave circuits, design, and measurements. Two major shortcomings of most current directional couplers used for coaxial, strip or microstrip transmission lines, or slab lines or similar guides and lines with TEM mode propagation are narrow bandwidth for isolation/directivity and large size. The size is dictated because quarter wave coupling is required on most designs. Another shortcoming is the lack of low loss couplers for coaxial lines, which is very important for many RF/microwave test and measurement systems such as load/source pull measurements.
It would be advantageous to provide couplers which overcome these difficulties by making the size of the coupler significantly small, the band width for isolation/directivity high and the losses low.